1. Field of the Invention
The present invention relates to a semiconductor device, a display device, and a light-emitting device, and a method for driving a semiconductor device, a display device, and a light-emitting device. Further, the present invention relates to an electronic device including the semiconductor device, the display device, or the light-emitting device.
2. Description of the Related Art
Display devices are used for a variety of electric products such as a mobile phone and a television receiver. As a display element used in a display device, a light-emitting element such as an EL element (an EL element containing an organic material and an inorganic material, an organic EL element, and an inorganic EL element) is favorable for improving the image quality because of high contrast ratio, high response speed to input signals, and wide viewing angle characteristics, and thus has been actively researched. Moreover, as for a display device including an EL element (hereinafter referred to as an EL display device), increase in screen size of the display device has been actively researched and developed.
In the EL display device, an EL element is driven in accordance with the amount of current flowing through the EL element. Accordingly, each pixel provided in a pixel portion, which is a display region in a display portion, is connected to a wiring for supplying current. The wiring for supplying current is formed using a wiring extended from the outside of the display region. Moreover, a TFT (thin film transistor) which is an element for controlling current supplied to the EL element is provided in each pixel of the pixel portion.
A TFT formed using polycrystalline silicon (polysilicon, hereinafter also referred to as p-Si) has high field effect mobility and excellent electric characteristics as compared to a TFT formed using amorphous silicon (hereinafter also referred to as a-Si), and thus is more suitable for a TFT used in an EL display device. On the other hand, TFTs formed using p-Si have a problem in that electric characteristics such as threshold voltage are likely to vary due to defects of bonding at crystal grain boundaries. Accordingly, as for a pixel including a TFT formed using p-Si, a structure including a circuit for compensating variation in threshold voltage of TFTs is disclosed (see Patent Documents 1 to 3).